Children with learning problems like dyslexia have brains that are two to four years behind in development compared with normal children, according to a new study. Northwestern University researchers say their finding may explain why learning-disabled children often toilet train late, talk later and appear less mature than other kids. The study also found that while lagging brains can catch up, many seem to stop developing altogether at puberty. Researchers say the finding confirms the need for early childhood intervention for learning disabilities.